What is the amulet in the movie Constantine?

Answer 1

It is a Triquetra. It was first seen in ancient Germanic paganism as a rune-stone similar to the Valknut and then in Celtic artwork as space filler or ornament. It's first known use as a religious symbol was with Christians during the Celtic revival of the 19th century. The symbol bore resemblance to the three-leafed shamrock that St. Patrick offered as a trinity symbol, so it became another symbol of the Holy Trinity (Father, Son, Holy Spirit).

Neopaganism has adopted the symbol to represent among many other things to symbolize the three stages of woman (Maiden, Wife, Crone). Wiccans generally use the symbol to represent the three parts of human existence (mind, body, soul).